化工流体流动与传热 (2)精选PPT.ppt

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1、化工流体流动与传热2023/4/121第1页，此课件共101页哦主讲主讲：余兰兰化工流体流动与传热化工流体流动与传热 (第二章第二章)2023/4/122第2页，此课件共101页哦New Words And Expressionsfluidshear stressesincompressiblecompressiblesolid boundariespotential flowirrotational flowfrictionmechanical energyboundary layervelocity-gradient shear-stress fields流体流体剪应力剪应力不可压缩的不可

2、压缩的可压缩的可压缩的固体壁面固体壁面势流势流无旋流无旋流摩擦摩擦机械能机械能边界层边界层速度梯度速度梯度剪应力场剪应力场2023/4/123第3页，此课件共101页哦turbulencevelocity fieldsteadyone-dimensional flowvectorcomponents scalareddieslaminar flowrheological behavior of fluidsNewtonian fluidsnon-Newtonian fluids 湍流湍流速度场速度场稳定的稳定的一维流动一维流动矢量矢量分量分量标量标量漩涡漩涡 层流层流流体流变性质流体流变性质牛

3、顿型流体牛顿型流体非牛顿型流体非牛顿型流体2023/4/124第4页，此课件共101页哦threshold shear stressBingham plasticspseudoplastic fluiddilatant fluidshear-rate-thinningshear-rate-thickeningtime-dependent flowthixotropic liquidsrheopectic substancesviscositycritical velocityturbulent flow剪切应力的临界值剪切应力的临界值宾哈姆塑性流体宾哈姆塑性流体假塑性流体假塑性流体涨塑性流体涨

4、塑性流体剪切速率递减剪切速率递减剪切速率递增剪切速率递增时变流时变流摇溶性液体摇溶性液体震凝性物质震凝性物质粘度粘度临界速度临界速度湍流湍流2023/4/125第5页，此课件共101页哦the diameter of the tubedensityaverage linear velocitygroupReynolds numbervelocity fluctuationsqualitative and quantitativewall turbulencefree turbulencediameterpotential energyboundary layersthe bulk fluid

5、velocitythe viscous sublayer管径管径密度密度平均线速率平均线速率群组群组雷诺准数雷诺准数脉动速度脉动速度定性和定量定性和定量壁面湍流壁面湍流自由湍流自由湍流直径直径势能势能边界层边界层主流体速度主流体速度粘滞底层粘滞底层2023/4/126第6页，此课件共101页哦缓冲层缓冲层湍流层湍流层过渡过渡湍流强度湍流强度过渡长度过渡长度二维的二维的速度梯度速度梯度动量动量 漩涡漩涡尾流尾流剪应力剪应力压力损失压力损失边界层分离边界层分离the buffer layerthe turbulent zonetransitionintensity of turbulencetra

6、nsition lengthtwo-dimensionalvelocity gradientsmomentumvorticeswakeshear stressespressure loss inboundary-layer separation2023/4/127第7页，此课件共101页哦the mass-balancecontinuity equationsthe linear-and angular-momentum-balance the mechanical-energy balance vectortensorstreamlines质量守恒质量守恒连续性方程式连续性方程式线性动量守恒

7、和角动量守恒线性动量守恒和角动量守恒机械能守恒机械能守恒矢量矢量张量张量流线流线2023/4/128第8页，此课件共101页哦 Chapter 2 Fluid Flow2.1 Fluid Statics and Its Applications 2023/4/129第9页，此课件共101页哦Nature of Fluids uAn attempt to change the shape of a mass of fluid results in layers of fluid sliding over one another until a new shape is attained l流体是

8、不能永久抵抗形变的物质。流体是不能永久抵抗形变的物质。l若想改变一定质量流体的形状，将导致流体中层与层之间若想改变一定质量流体的形状，将导致流体中层与层之间相互滑动直至获得新的形状。相互滑动直至获得新的形状。uA fluid is a substance that does not permanently resist distortion2023/4/1210第10页，此课件共101页哦uDuring the change in shape，shear stresses exist，the magnitudes of which depend upon the viscosity of th

9、e fluid and the rate of sliding，but when a final shape has been reached，all shear stresses will have disappeared u在变形时，存在剪应力，其大小取决于流体的黏在变形时，存在剪应力，其大小取决于流体的黏度和滑动的速率，当最终形状确定后，剪应力消度和滑动的速率，当最终形状确定后，剪应力消失。失。2023/4/1211第11页，此课件共101页哦v处于平衡的流体与剪应力无关。处于平衡的流体与剪应力无关。vA fluid in equilibrium is free from shear s

10、tressesv在给定的温度和压力下，流体的密度是常数，在工程上通常以磅在给定的温度和压力下，流体的密度是常数，在工程上通常以磅/立方英尺或公斤立方英尺或公斤/立方米来测量。立方米来测量。vAt a given temperature and pressure，a fluid possesses a definite density，which in engineering practice is usually measured in pounds per cubic foot or kilograms per cubic meter 2023/4/1212第12页，此课件共101页哦vIf

11、 the density is but little affected by moderate changes in temperature and pressure，the fluid is said to be incompressible，and if the density is sensitive to changes in these variables，the fluid is said to be compressible v如果温度和压力轻度改变时对密度的影响很小，将该流体称为如果温度和压力轻度改变时对密度的影响很小，将该流体称为不可压缩的；若密度受这些变量的影响很大，则称该

12、流体为可不可压缩的；若密度受这些变量的影响很大，则称该流体为可压缩的。压缩的。2023/4/1213第13页，此课件共101页哦2.2 Fluid-Flow Phenomena 流动流体的行为在很大程度上取决于流体是否受固流动流体的行为在很大程度上取决于流体是否受固体边界的影响。体边界的影响。The behavior of a flowing fluid depends strongly on whether or not the fluid is under the influence of solid boundaries.1.Potential flow 2.2.1 Fluid flow

13、2023/4/1214第14页，此课件共101页哦A moving fluid uninfluenced by stationary solid walls is not subject to shear,and shear stresses do not exist within it.不受固定固体壁面影响的流动流体就不受剪切力作用，不受固定固体壁面影响的流动流体就不受剪切力作用，且其内部不存在剪切应力。且其内部不存在剪切应力。2023/4/1215第15页，此课件共101页哦The flow of incompressible fluids with no shear is called

14、potential flow and is completely described by the principles of Newtonian mechanics and conservation of mass.无剪切力的不可压缩流体的流动称为无剪切力的不可压缩流体的流动称为“势流势流”，完全由牛顿力学和，完全由牛顿力学和质量守恒原理来描述。质量守恒原理来描述。vThe mathematical theory of potential flow is highly developed but is outside the scope of this bookv势流的数学理论已被深入研究，

15、但不属于本书的范围。势流的数学理论已被深入研究，但不属于本书的范围。2023/4/1216第16页，此课件共101页哦Potential flow has two important characteristics:v(1)neither circulations nor eddies can form within the stream,so that potential flow is also called irrotational flow,and v(2)friction cannot develop,so that there is no dissipation of mechan

16、ical energy into heat.v势流有两个重要的特征：势流有两个重要的特征：v（2）没有摩擦，因此没有机械能损耗变热的情况。）没有摩擦，因此没有机械能损耗变热的情况。v（1）在流动流体内不能生成漩涡，因此势流也称为无旋流）在流动流体内不能生成漩涡，因此势流也称为无旋流2023/4/1217第17页，此课件共101页哦vA fundamental principle of fluid mechanics,originally stated by Prandtl in 1904,is that except for fluids moving at low velocities or

17、 possessing high viscosities,the effect of the solid boundary on the flow is confined to a layer of the fluid immediately adjacent to the solid wall.vPrandtl 在在1904年最先提出的流体力学基本原理中说，除低速年最先提出的流体力学基本原理中说，除低速流动或黏度很大的流体外，固体边界对流动的影响仅限于紧挨流动或黏度很大的流体外，固体边界对流动的影响仅限于紧挨固体壁面的流体层内。固体壁面的流体层内。2.Boundary layer2023/4

18、/1218第18页，此课件共101页哦v此流体层称为边界层，剪切力仅存在于这部分流体中。在此流体层称为边界层，剪切力仅存在于这部分流体中。在边界层之外，则是势流。边界层之外，则是势流。vThis layer is called the boundary layer,and shear forces are confined to this part of the fluid.Outside the boundary layer,potential flow survives.vMost technical flow processes are best studied by consideri

19、ng the fluid stream as two parts,the boundary layer and the remaining fluid.v对于大多数工艺流动过程，是将流体看作两部分对于大多数工艺流动过程，是将流体看作两部分边界层及边界层及其以外的流体，进行了深入的研究。其以外的流体，进行了深入的研究。2023/4/1219第19页，此课件共101页哦vIn some situations,the boundary layer may be neglected,and in others,such as flow through pipes,the boundary layer

20、fills the entire channel,and there is no potential flow.v在在某某些些情情况况下下可可以以忽忽略略边边界界层层，而而在在另另外外一一些些时时候候边边界界层层充充满整个通道（像通过管道的流动），就没有势流。满整个通道（像通过管道的流动），就没有势流。vWithin the current of an incompressible fluid under the influence of solid boundaries,four important effects appear:v在在受受固固体体边边界界影影响响的的不不可可压压缩缩流流体体

21、的的流流动动中中，有有四四个个重重要要的的影影响因素：响因素：2023/4/1220第20页，此课件共101页哦l(1)the coupling of velocity-gradient and shear-stress fields,v（1）速度梯度与剪应力场的关系；）速度梯度与剪应力场的关系；v（2）湍流的开始；）湍流的开始；v（3）边界层的生成与发展；）边界层的生成与发展；v（4）边界层的分离。）边界层的分离。v(2)the onset of turbulence,v(3)the formation and growth of boundary layers,andv(4)the sep

22、aration of boundary layers from contact with the solid boundary.2023/4/1221第21页，此课件共101页哦3.The Velocity FielduWhen a stream of fluid is flowing in bulk past a solid wall，the fluid adheres to the solid at the actual interface between solid and fluid u当流体大量流过固体壁面时，与固体紧挨着的流体粘附当流体大量流过固体壁面时，与固体紧挨着的流体粘附在固

23、体表面上。在固体表面上。2023/4/1222第22页，此课件共101页哦uThe adhesion is a result of the force fields at the boundary，which are also responsible for the interfacial tension between solid and fluid v这个粘附是力场在边界作用的结果，也与固体和流体间的界面拉这个粘附是力场在边界作用的结果，也与固体和流体间的界面拉力有关。力有关。vIf，therefore，the wall is at rest in the reference frame

24、chosen for the solid-fluid system，the velocity of the fluid at the interface is zero u因此，若认为固体壁面相对于流体是静止的，则在界面上流体的因此，若认为固体壁面相对于流体是静止的，则在界面上流体的速度为零。速度为零。2023/4/1223第23页，此课件共101页哦uSince at distances away from the solid the velocity is finite，there must be variations in velocity from point to point in

25、the flowing stream v因为距固体壁面一定距离处的速度是一定值，所以在流体中的速度因为距固体壁面一定距离处的速度是一定值，所以在流体中的速度逐点变化。逐点变化。vTherefore，the velocity at any point is a function of the space coordinates of that point，and a velocity field exists in the space occupied by the fluid v因此，任一点的流速是该点空间坐标的函数，且在流体中存在速因此，任一点的流速是该点空间坐标的函数，且在流体中存在速度场

26、。度场。2023/4/1224第24页，此课件共101页哦4.Laminar FlowvAt low velocities fluids tend to flow without lateral mixing，and adjacent layers slide past one another like playing cards v流体在低速流动时，没有横向混合，相邻层间就像扑克牌那样流体在低速流动时，没有横向混合，相邻层间就像扑克牌那样互相滑过。互相滑过。2023/4/1225第25页，此课件共101页哦vThere are neither cross-currents nor eddie

27、sv既没有横向流也没有漩涡。既没有横向流也没有漩涡。vThis regime is called laminar flowv这种情况称为层流。这种情况称为层流。vAt higher velocities turbulence appears，and eddies form，which，as discussed later，lead to lateral mixingv在较高流速下，出现湍流，形成引起横向混合的漩涡，将在后在较高流速下，出现湍流，形成引起横向混合的漩涡，将在后面讨论。面讨论。2023/4/1226第26页，此课件共101页哦2.2.2 Newtonian and non-Newt

28、onian FluidsvThe fact that at each point in a flowing fluid both a rate of shear and a shear stress exist suggests that these quantities may be related.v在流动流体中任一点上同时存在速度梯度和剪切应力这个在流动流体中任一点上同时存在速度梯度和剪切应力这个事实提醒我们这些量之间可能是有联系的。事实提醒我们这些量之间可能是有联系的。1.Newtonian and non-Newtonian Fluids2023/4/1227第27页，此课件共101

29、页哦vIn fact they are,the science of rheology deals with their coupling and the complex relations between them.v事实上的确如此，流变学论述它们之间的联系以及复杂关系。事实上的确如此，流变学论述它们之间的联系以及复杂关系。vFigure 2-1 shows several examples of the rheological behavior of fluids.v图图2-1示出流体流变性质的几个例子。示出流体流变性质的几个例子。2023/4/1228第28页，此课件共101页哦Figu

30、re 2-1 Shear stress versus velocity gradient for Newtonian and non-Newtonian fluids2023/4/1229第29页，此课件共101页哦vThe curves are plots of shear stress vs.rate of shear and apply at constant temperature and pressure.v这这些曲些曲线线是剪切是剪切应应力力对对速度梯度作速度梯度作图图的的结结果，适用于恒果，适用于恒温温恒恒压条压条件。件。vThe simplest behavior is tha

31、t shown by curve A,which is a straight line passing through the origin.v最简单的特性如曲线最简单的特性如曲线A所示，这是一条过原点的直线所示，这是一条过原点的直线2023/4/1230第30页，此课件共101页哦lFluids following this simple linearity are called Newtonian fluids.vGases,true solutions,and noncolloidal liquids are Newtonian.v气体、实际溶液以及非胶体液体都是牛顿型的。气体、实际溶液

32、以及非胶体液体都是牛顿型的。v遵循这个简单直线的流体称为牛顿型流体。遵循这个简单直线的流体称为牛顿型流体。2023/4/1231第31页，此课件共101页哦v图图2-1中其他曲线表示非牛顿型流体的流变性质。中其他曲线表示非牛顿型流体的流变性质。vThe other curves shown in Fig.2-1 represent the rheological behavior of liquids called non-Newtonian.lSome liquids,e.g.,sewage sludge,do not flow at all until a threshold shear

33、stress,denoted by ,is attained and then flow linearly at shear stresses greater than .v某些液体，如污泥浆，在其剪切应力达到临界值（记作某些液体，如污泥浆，在其剪切应力达到临界值（记作0）以）以前根本不流动，超过此值，流动中剪切应力则从前根本不流动，超过此值，流动中剪切应力则从0起线性增长。起线性增长。2023/4/1232第32页，此课件共101页哦lCurve B is an example of this relation.Liquids acting this way are called Bingh

34、am plastics.l曲线曲线B是这个关系的例子。这类液体称为宾哈姆塑性流体。是这个关系的例子。这类液体称为宾哈姆塑性流体。vLine C represents a pseudoplastic fluid.vC线表示假塑性（或拟塑性）流体。线表示假塑性（或拟塑性）流体。vThe curve passes through the origin,is concave downward at low shears,and becomes linear at high shears.v该线通过原点，在低剪切力时凹边向下，高剪切力时则成直线。该线通过原点，在低剪切力时凹边向下，高剪切力时则成直线。2

35、023/4/1233第33页，此课件共101页哦lRubber latex is an example of such a fluid.l乳胶就是这样一种流体。乳胶就是这样一种流体。lCurve D represents a dilatant fluid.The curve is concave upward at low shears and becomes linear at high shears.l曲线曲线D代表涨塑性流体，在低剪切力时凹边向上，高剪切力时也代表涨塑性流体，在低剪切力时凹边向上，高剪切力时也成直线。成直线。2023/4/1234第34页，此课件共101页哦lQuicks

36、and and some sand-filled emulsions show this behavior.l流沙以及某些充满沙子的乳浊液表现出这种性质。流沙以及某些充满沙子的乳浊液表现出这种性质。vPseudo plastics are said to be shear-rate-thinning and dilatant fluids shear-rate-thickening.v假假塑塑性性流流体体称称为为剪剪切切速速率率递递减减型型流流体体，涨涨塑塑性性流流体体称称为为剪剪切切速速率递增型流体。率递增型流体。2023/4/1235第35页，此课件共101页哦2.Viscosity lI

37、n a Newtonian fluid the shear rate obviously is proportional to the shear stress.l在牛顿型流体中，剪切速率显然是与剪切应力成正比的。在牛顿型流体中，剪切速率显然是与剪切应力成正比的。2023/4/1236第36页，此课件共101页哦lThe proportionality constant is called the viscosity and is defined by the equationl比例常数比例常数称为黏度，其定义由方程（称为黏度，其定义由方程（2-1）给出：）给出：（2-1）2023/4/123

38、7第37页，此课件共101页哦3.TurbulencelThe distinction between laminar and turbulent flow was first demonstrated in a classic experiment by Osborne Reynolds,reported in 1883.l层流与湍流的区别由层流与湍流的区别由Osborne Reynolds在在1883年首先从试验中年首先从试验中发现。发现。vThe equipment used by Reynolds is shown in Fig.2-2.vReynolds所用的设备示于图所用的设备示于

39、图2-2。(1)Turbulence2023/4/1238第38页，此课件共101页哦 Figure 2-2 Reynolds experiment2023/4/1239第39页，此课件共101页哦vReynolds found that,at low flow rates,the jet of colored water flowed intact along with the mainstream and no cross mixing occurred.lReynolds发现，在低流速下，染色水流与主流体一起流动，发现，在低流速下，染色水流与主流体一起流动，不受任何影响，不发生横向混合。

40、不受任何影响，不发生横向混合。lThe behavior of the color band showed clearly that the water was flowing in parallel straight lines and that the flow was laminar.l染色水的特征表明水是以平行直线的方式流动，即层流。染色水的特征表明水是以平行直线的方式流动，即层流。2023/4/1240第40页，此课件共101页哦vWhen the flow rate was increased,a velocity,called the critical velocity was

41、reached at which the thread of color disappeared and the color diffused uniformly throughout the entire cross section of the stream of water.l当流速增加，达到一个临界速度时，染色水流消失，而颜色在当流速增加，达到一个临界速度时，染色水流消失，而颜色在整个水流中均匀地扩散。整个水流中均匀地扩散。2023/4/1241第41页，此课件共101页哦lThis behavior of the colored water showed that the water

42、 no longer flowed in laminar motion but moved erratically in the form of crosscurrents and eddies.l染色水流的这个特征表明水不再是层流流动，而是以涡流和漩染色水流的这个特征表明水不再是层流流动，而是以涡流和漩涡的形式不稳定地移动。涡的形式不稳定地移动。vThis type of motion is turbulent flow.l这个运动类型称为湍流。这个运动类型称为湍流。2023/4/1242第42页，此课件共101页哦 (2)Reynolds Number and Transition fro

43、m Laminar to Turbulent Flow2023/4/1243第43页，此课件共101页哦uReynolds studied the conditions under which one type of flow changes into the other and found that the critical velocity,at which laminar flow changes into turbulent flow,depends on four quantities:vReynolds研究了流动类型转变的条件，并发现了临界速度，在此研究了流动类型转变的条件，并发现

44、了临界速度，在此值下层流变为湍流。值下层流变为湍流。v该值与该值与4个量有关：管径、液体的黏度、密度和平均线速度。个量有关：管径、液体的黏度、密度和平均线速度。vthe diameter of the tube,and the viscosity,density,and average linear velocity of the liquid.2023/4/1244第44页，此课件共101页哦lFurthermore,he found that these four factors can be combined into one group and that the change in k

45、ind of flow occurs at a definite value of the group.l进而，他又发现这进而，他又发现这4个因素可被组成一个数群，该数群在某一定值个因素可被组成一个数群，该数群在某一定值时，流动的类型发生变化。时，流动的类型发生变化。vThe grouping of variables so found wasv这些变量的组合形式为这些变量的组合形式为2023/4/1245第45页，此课件共101页哦 （2-2）where d=diameter of tube u=average velocity of liquid =density of liquid =v

46、iscosity of liquid2023/4/1246第46页，此课件共101页哦vThe dimensionless group of variables defined by Eq.(2-2)is called the Reynolds number，Re.v由方程（由方程（2-2）定义的这些变量的无因次数群称为雷）定义的这些变量的无因次数群称为雷诺准数。诺准数。v It is one of the named dimensionless groups listed in Appendix 5.v它是附录它是附录5中所列的无因次数群之一。中所列的无因次数群之一。2023/4/1247第

47、47页，此课件共101页哦vIts magnitude is independent of the units used,provided the units are consistent.v它的值与所使用的单位制无关，只要单位一致即可。它的值与所使用的单位制无关，只要单位一致即可。vAdditional observations have shown that the transition from laminar to turbulent flow actually may occur over a wide range of Reynolds numbers.v另外的观察表明，由层流到湍

48、流的过渡可以发生在一个较大的雷诺数另外的观察表明，由层流到湍流的过渡可以发生在一个较大的雷诺数范围内。范围内。2023/4/1248第48页，此课件共101页哦vLaminar flow is always encountered at Reynolds numbers below 2,100,but it can persist up to Reynolds numbers of several thousand under special conditions of well-rounded tube entrance and very quiet liquid in the tank.v

49、雷诺数低于雷诺数低于2100时，总是层流流动，但在管口很圆，箱内液体很时，总是层流流动，但在管口很圆，箱内液体很平静的特殊条件下，在雷诺数达到几千时仍可保持层流。平静的特殊条件下，在雷诺数达到几千时仍可保持层流。2023/4/1249第49页，此课件共101页哦vUnder ordinary conditions of flow,the flow is turbulent at Reynolds numbers above about 4,000.v在通常的流动条件下，雷诺数大于在通常的流动条件下，雷诺数大于4000时就是湍流流动。时就是湍流流动。v Between 2,100 and 4,0

50、00 a transition,or dip,region is found,where the type of flow may be either laminar or turbulent,depending upon conditions at the entrance of the tube and on the distance from the entrance.v在在2100和和4000之之间间是是过过渡渡区区，流流动动类类型型不不是是层层流流就就是是湍湍流流，取取决决于于管口的条件以及到管口的距离。管口的条件以及到管口的距离。2023/4/1250第50页，此课件共101页哦v